1. Field of the Invention
The present invention relates to methods for obtaining shear wave velocities and, more particularly, to a method for obtaining by wellbore measurements shear wave velocities in formations where the formation shear wave velocity is less than the acoustic velocity of the fluid conventionally used within the wellbore.
2. Setting of the Invention
Sonic wave train logging is used to obtain compressional and shear wave velocity information for seismic exploration and geologic evaluation purposes. This logging technique often utilizes sonic frequencies of about 5 to 50 kHz and also often utilizes acoustic monopole source and receiver arrays placed within a wellbore. Source and receiver arrays are utilized to measure the formation's elastic properties via acoustic-elastic energy mode conversions at the wellbore fluid-formation interface. The sonic or acoustic monopole energy transmitting device generates acoustic compressional waves, which are converted at the borehole wall to both compressional and shear elastic waves critically refracted in the formation.
A problem often encountered in the field is that the formation to be logged has a shear wave velocity which is less than the acoustic velocity of the fluid within the wellbore. In such instances, as is well known, no critically refracted formation shear wave is generated via the acoustic-elastic mode conversion at the wellbore fluid-formation interface, thus no shear wave can be detected by the receivers.
Various solutions to this important problem have been proposed. One approach is to place acoustic, multipole source-receiver arrays in the wellbore which directly impart and sense shearing motion at the wellbore wall without relying upon acoustic elastic mode conversion. A problem with using such multipole source-receiver arrays is the requirement of azimuthal alignment and centering of the receivers and the transmitters.
Another approach relies upon other modes of borehole acoustic energy propagation, such as Stoneley or "tube-waves". These waves are generated even when the formation shear velocity is less than the borehole fluid velocity and can be recorded and analyzed to infer formation shear velocity via known theoretical relationships. The analysis procedures used to "back out" the formation shear velocity from the Stoneley or "tube-waves" are computationally intensive and, thus, it is difficult to generate the shear wave velocity on a "real time" basis while logging. This "real time" generation of the signals is extremely important in the timely utilization of the logging information. Further, these computational methods to back out the shear velocity require exceptionally good data quality to yield stable numerical results.
All of the above-described approaches for solving the problem require either extensive modifications to the tools normally utilized in the field, or extensive and expensive computing capability. The inventors hereof know of no reference that discloses or suggests intentionally altering the acoustic velocity properties of the wellbore fluid to solve the above problem.